Iowa law requires that pumps be labeled with the ethanol mixture.
Nearly every station offers an ethanol-free option, amazing considering the power of our farm/ethanol lobby.
Octane rating can also be a clue, but it is not a sure thing.
Generally our "regular" is Octane 87, then add 10 percent ethanol, you get octane 89. Much of the premium - 92,93 is ethanol free.
Some chains mix octane 85 (mountain altitude gas) with 10 percent ethanol to give 87. This has to be the worst possible choice, 10 percent ethanol and still low octane.
Yes, I know octane 85 is just fine at high altitude... that is not relevant in any part of Iowa!! We ski off the top of haystacks and manure piles here.
Like others here, I avoid ethanol as much as possible.